Add function descriptions

[dbanck]

This PR adds descriptions for all Terraform functions and is the first step in enabling machine-readable function signatures (more: TF-508: Machine-readable function signatures).

It is planned to export the descriptions via a new terraform metadata functions --json command. The first consumer of the JSON output will be the Terraform language server, to provide function signature information inside the editor. After that, the docs website might be another potential consumer, making the description list the single source of truth.

Instead of iterating over the functions list and using WithDescription for each one, I've another approach, referring to description entries from a function definition. But that approach requires edits in multiple places and matching slice indices when one wants to add a parameter description.

Target Release

1.4.x

[kmoe]

LGTM

[github-actions]

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[github-actions]

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